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ISSN 1392–1320 MATERIALS SCIENCE (MEDŽIAGOTYRA). Vol. 19, No. 2. 2013

Changes in Storage Properties of Hydrides Induced by Ion Irradiation

Jasmina GRBOVIĆ NOVAKOVIĆ 1 ∗, Sandra KURKO 1, Željka RAŠKOVIĆ-LOVRE 1, Sanja MILOŠEVIĆ 1, Igor MILANOVIĆ 1, Zoran STOJANOVIĆ 2, Radojka VUJASIN 1, Ljiljana MATOVIĆ 1

1 Vinča Institute of Nuclear Sciences, University of Belgrade P.O.Box 522, 11000 Belgrade, Serbia 2 Institute of Technical Sciences of SASA, Knez Mihailova 35/IV, 11000 Belgrade, Serbia

http://dx.doi.org/10.5755/j01.ms.19.2.1579

Received 20 April 2012; accepted 13 January 2013

The influence of structural changes caused by irradiation with different ions, their energies and fluences on sorption properties has been investigated. The irradiation has been done using B3+ and N3+ ions at 45 keV with ion fluence of 1016 ion/cm2 at the FAMA ion source at Vinča Institute of Nuclear Sciences in Belgrade. Morphology and microstructure of samples were analysed using XRD and particle size analysis, while estimation of penetration depth and deposition of defects were done by SRIM calculations. Hydrogen desorption properties and kinetics were evaluated from TPD measurements and numerical non-isothermal procedure. Results suggest that there are several mechanisms of desorption depending on defect concentration, their interaction and ordering. It has been also demonstrated that the changes in near-surface area play the crucial role in hydrogen desorption kinetics. It is confirmed that there is possibility to control the thermodynamic parameters of these systems by controlling vacancies depth profile and concentration. Keywords: hydrogen storage, MgH2, ion irradiation, TPD, non- isothermal kinetics.

1. INTRODUCTION

∗

In order to improve MgH2 storage properties i. e. to decrease the desorption temperature and improve desorption kinetics many efforts have been made [1 – 10]. The concept of metal hydride destabilization (including MgH2) relies on the capability to identify compounds, elements or methods that can lead to reduction of the reaction enthalpy. This concept is very general and therefore a large number of potential destabilization methods exists [1 – 10]. The commonly used methods are nanostructuring by decrease of particles size to nanometer level by ball milling [1 – 3] and nanostructuring with addition of impurities and/or catalysts [4 – 6]. Both destabilization methods can be declared as bulk modifications since all defects are introduced within whole volume of the sample. Those methods provide some improvements in kinetics and desorption temperature, but the reaction mechanism is still not clear. For example, the rate-limiting step for desorption from MgH2-Nb2O5 composite changes with increasing of milling time or Nb2O5 content, from chemisorption to interface velocity of the transformed phase [4]. According to Schulz et al. who investigated MgH2-V composites, the hydrogen desorption at high temperature and under high driving force, is controlled by the interface (Mg/MgH2) motion [5]. When the driving force is small, the early stage of hydrogen desorption is controlled by nucleation and growth and the later stage is controlled by long range hydrogen diffusion. At high temperatures, the rate-limiting step changes from interface control (before annealing) to surface control (after annealing), while at low temperatures, the rate-limiting step of desorption does not change after annealing.

∗

Corresponding author. Tel.:+381-11-3408552, fax: +381-11-3408224.

E-mail address: jasnag@vinca.rs (J.Grbović Novaković)

Therefore, it is quite difficult to understand what actually govern the process especially when one have in mind the fact that the sorption of MgH2 can be describe by several steps:

(a) Hydrogen in the gas phase transfer to the surface of the metal particle;

(b) Hydrogen diffusion through the boundary layer between gas phase and solid particle;

(c) Physisorption of hydrogen molecules on the solid surface;

(d) Dissociation of hydrogen molecules and chemisorptions;

(f) Surface penetration of hydrogen atoms; (g) Diffusion of hydrogen atoms through the hydride

product layer to the hydride/metal interface and (h) Chemical reaction and nucleus formation (hydride

production) Any of them could be the rate limiting step [10]. In order to reveal the most important factors governing

the sorption properties of MgH2, it is crucial to obtain a better understanding of the changes induced by the destabilization processes [7 – 11]. We have investigated the influence of well defined structural changes introduced by ion irradiation within surface layer of MgH2. Understand-ing of the relations between concentration and type of produced defects and MgH2 sorption properties will help to overcome some of the material’s drawbacks and will provide the possibility to investigate stability of particular induced modifications during desorption.

2. EXPERIMENTAL DETAILS

The commercial (Alfa Aeser) MgH2 powder was pressed on the Al foil to obtain thin layer and homogeneously irradiated using B3+ and N3+ ions at 45 keV with ion fluence of 1016 ion/cm2. The irradiation

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was done at FAMA ion source at Vinča Institute. Based on numerical simulations and consequent analysis of depth profile and energy loss mechanisms, 45 keV is found to be an optimal energy to study the near surface effects. The reversibility of the irradiated material wasn’t an issue of this study.

Morphological and microanalytical characterization was carried out by VEGA TS 5130MM, Tescan Brno SEM equipped with EDS detector. Malvern 2000SM Mastersizer laser scattering particle size analysis system was used to obtain the quantitative MgH2 particle size distribution. X-ray diffraction analysis was used to identify the crystalline phases and lattice parameters as well as the crystalline size and strain of irradiated samples by means of Siemens Kristalloflex D-500 device, with Cu-Kα Ni filtrated radiation (λ = 1.5406 Å). Lattice parameters were refined from the fitted data using the least square procedure. Standard deviation obtained was 1 %. Williamson-Hall plots were used to separate the effects of the size and strain in the nanocrystals. Thermal behaviour of the samples was studied by TPD measurements at a constant heating rate of 5 K/min, from room temperature to 973 K, under starting vacuum of 3×10−6 mbar, utilizing homemade equipment, with a quadruple mass spectrometer EXTORR XT300. In order to deduce the mechanism of thermal decomposition process [10, 12 – 14] we have used non-isothermal approach under the assumption that the non-isothermal reactions proceed isothermally at an

infinitesimal time interval, so that the reaction ratedt

dθ,

can be expressed by Arrhenius type equation:

( )θθ

fAedt

dRT

Edes−

= , (1)

where A is the pre-exponential factor, θ is degree of conversion, f(θ

) depends on the mechanism of the process

involved. Substitution of linear heating rate, β , dt

dT=β

into Eq. (1) gives:

( )dTe

A

f

dRT

Ea

des−

=

βθ

θ. (2)

To evaluate the kinetic parameters from the previous equations, one can plot:

( )⎟⎠

⎞⎜⎝

⎛=⎥

⎦

⎤⎢⎣

⎡

Tf

T

g 1000ln

2

θ. (3)

a

desE and A can be calculated from the slope and intercept

respectively. Therefore, one can obtain the mechanism by searching for the best fit of the experimental data.

3. RESULTS AND DISSCUSION

3.1. Numerical analysis of irradiation effects

Effects of 45 keV of B3+ and N3+ ions irradiation of MgH2 samples are estimated by Monte Carlo simulations with statistics of 10 000 ion events, using the detailed calculation with full damage cascades option of SRIM 2008 code. The main results are presented in Fig. 1, a and b, and Table 1. The B range distribution maximum is found at the depth of 216 nm with the full width at the half of the maximum (FWHM) of 93 nm, indicating a relatively large dispersion of the B3+ ions in the samples. A maximum of recoiled Mg and H atoms range is found to be around 167 nm (see Fig. 1, a). Prevalent direct N3+ energy loss mechanism is ionization, while direct vacancies production

a 45 keV B b 45 keV N

Fig. 1. Monte Carlo simulations of irradiation effects of 45 keV B (a) and N (b) on MgH2 matrix obtained by SRIM 2003 package: top – depth distribution of incident ions and recoiled atoms; bottom – vacancies production as function of depth

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(primary knock-outs) participates with only 0.55 % in total only 1.7 %. The rest of energy is transferred to Mg and H recoils and consecutive cascades with quite different energy loss mechanisms contribution ratio. Similar situation holds for boron irradiation. Recoils loose energy mostly through two competitive processes, ionization and phonon excitation in both cases. The most of incident energy is transferred to recoils (32 % for B compared to 42 % in N case) and consecutive cascades. Further, total number of vacancies per incident B3+ ion is found to be 288, which is considerably less than the number of vacancies caused by incident N3+ ion of same energy (359 vacancies per ion). Obviously, 45 keV of B3+ ions produce less defects, they are placed deeper and spread over a larger distance in the sample, than those produced by corresponding 45 keV N3+ irradiation (see Table 1, Fig. 1, b). This is probably the reason for different mechanism and temperature of desorption.

3.2. Morphological and microstructural evaluation

of irradiation effects

Sample irradiated with 45 keV of N3+ ions (N16) show the morphology quite similar to non treated MgH2 with

agglomerates from 10 μm to 100 μm (Fig. 3, b). Anyhow, figure at higher magnification (Fig. 3, c) shows the typical morphology for irradiated samples i. e. cracks leading to quite damaged onion like structure [7]. On the other hand, agglomerates of sample irradiated with 45 keV of B3+ (B16) are much smaller, from 2 μm to 20 μm (Fig. 3, a). This is in agreement with the results of particle size analysis (Fig. 3, Table 2).

Both samples show the monomodal particle size distribution, but with the onset at lower sizes, which is more pronounced for sample irradiated with boron. At first sight, the results of SEM and particle size analysis seem to be ambiguous, but one should have in mind the fact that

particle size analysis was done under constant ultrasound regime since the particles tend to agglomerate.

XRD patterns (Fig. 4) of commercial sample (AA) show narrow peaks at 2θ = 27.77 (110), 35.77 (101), 40 (200) 55 (211), characteristic for β-MgH2 tetragonal structure and minor secondary phase (metallic Mg at 2θ = 32.35 and MgO at 2θ = 36.77). The irradiated samples exhibit the broadening of typical β-MgH2 peaks, pointing out a strong influence of ion irradiation on lattice parameters, crystallite size and microstrain (see Table 2).

Table 1. Results of SRIM simulations of 45 keV B3+ and N3+ ions irradiation of MgH2 matrix

Species Range maximum

position [nm] Ion range FWHM

[nm] Vacancies

(total per ion)

Energy loss [%]

Ionisation Vacancies production

Phonon exitation

N Ion 170 90

359 55.58 0.55 1.78

Recoils 125 159 17.53 1.84 22.72

B Ion 216 93

288 64.86 0.50 1.80

Recoils 160 193 13.35 1.41 18.07

a b c

Fig. 2. SEM micrographs of sample irradiated with 45 keV of B3+ (a) and N3+ ions (N16) using ion fluence of 1016 ion/cm2 (b) sample N16 marker 20 μm (c) sample N16 marker 10 μm

Table 2. Structural parameters obtained from XRD and particle size analysis of commercial MgH2 powder (AA) and samples irradiated

with 45 keV B3+ (B16) and 45 keV N3+(N16) using ion fluence of 1016 ion/cm2

Sample Crystallite size

[nm] Microstrain

Average particle size [μm]

Lattice parameter of β-MgH2 [Å]

a c V

AA 83 1.4·10–3 38 4.5168 3.0205 61.623

B16 76 1.3·10–3 21 4.4915 3.0113 60.744

N16 57 4.98·10–4 27 4.5132 3.0191 61.498

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Fig. 3. Particle size analysis of commercial MgH2 powder (AA) and and samples irradiated with 45 keV B3+ (B16) and 45 keV N3+(N16) using ion fluence of 1016 ion/cm2

Fig. 4. XRD paterns of commercial MgH2 powder (AA) and samples irradiated with 45 keV B3+ (B16) and 45 keV N3+(N16) using ion fluence of 1016 ion/cm2

In both irradiated samples, some Mg(OH)2 has been formed, but the quantity of hydroxide is much higher in sample were the defects are deposited closer to the surface and where the quantity of defects per incident ion is much higher (sample N16). On the other hand, amorphization is evident in the sample where the defects are positioned deeper into the bulk of sample (sample B16). Further, pronounced decrease of crystallite size is observed for samples irradiated with nitrogen (Table 2). The oxygen contamination during milling process or during XRD or SEM analysis is practically impossible to avoid, even though all experiments has been done under Ar atmosphere.

3.3. Desorption properties and kinetics

Hydrogen desorption properties and kinetics were evaluated from TPD measurements and numerical

procedure explained in experimental part (see Fig. 5) [10]. It can be noticed that all the samples completely release hydrogen in a single process. Commercial MgH2 (AA) releases hydrogen around 710 K while desorption temperature is reduced in both irradiated samples: 661 K for samples irradiated with N3+ (N16) and 678 K for sample irradiated with B3+ (B16).

The higher temperature of desorption obtained from the samples irradiated with B3+ ions, confirm the presumption that both, higher concentration of defects and their distribution closer to the sample surface, enhance the hydrogen desorption rate [8, 10].

Fig. 5. TPD spectra obtained at heating rate of 5 K/min of commercial MgH2 sample (AA) and samples irradiated with 45 keV B3+ (B16) and 45 keV N3+(N16) using ion fluence of 1016 ion/cm2

The problem of the reaction mechanism is in fact a subject of extensive debate, although grate amount of literature is available [1, 2, 4 – 6, 10 – 20]. Huot et al. suggested that nucleation and growth process takes place during desorption [15].. They found that Avrami parameter in JMA is 3 and interpreted this value as an instantaneous nucleation followed by an interface-controlled three-dimensional growth process. According to Wu et al. [16] the rate-controlling step is the recombination of H2 on the MgH2 surface. Leardini et al. indicated that 3D-growth of Mg phase controls the kinetics of MgH2 decomposition [17]. Ranjbar et al. show that hydrogen desorption reaction for pure MgH2 is controlled by recombination of chemisorbed hydrogen atoms (surface controlled reaction) [18]. However, upon Ni addition, which is good catalyst for hydrogen molecule dissociation and recombination, the hydrogen desorption reaction is controlled by bulk nucleation and three-dimensional growth of the existing Mg nuclei (JMA 3D) [17]. Liang et al. [19] and Hanada et al. [20], claimed that two-dimensional growth of existing nuclei (JMA 2D) is the rate-limiting step for their Ni doped Mg samples. The results obtained by Barkhordarian et al. indicate that hydrogen diffusion through the dehydrided MgH2 and through the grain boundaries of the magnesium hydride phase is comparably fast and not rate limiting if Nb2O5 is added as catalyst [4]. They have also demonstrated that the rate-limiting step for desorption from MgH2-Nb2O5 composite changes, with increasing of milling time or Nb2O5

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content, from chemisorption to interface velocity of the transformed phase [4].

On the other hand, controlling the depth of deposition and the number of defects, by ion irradiation, it is possible to unambiguously determine the reaction mechanism. The obtained rate limiting steps of desorption reaction for irradiated material and pure MgH2 are listed in Table 3. Regarding the untreated sample and B16, the best fit is obtained for phase boundary reaction with spherical symmetry (n = 3). On the other hand there is change in kinetics obtained for material irradiated with nitrogen. This fact confirms the presumption that the reaction mechanism depends on quantity and deposition depth of defects.

Table 3. Summary of results obtained after fitting of experimental data (Fig. 6: normalized TPD data)

Sample Reaction model Equation ⎟⎠

⎞⎜⎝

⎛

mol

kJdes

aE

AA 0.3 < θ < 0.9

Avrami Erofeev ( )[ ]31

1ln θ−− 372

N16 0.3 < θ < 0.9

Avrami Erofeev ( )[ ]4

1

1ln θ−−

253

B16 0.3 < θ <0.9

Avrami Erofeev ( )[ ]31

1ln θ−− 373

Fig. 6. Temperature evolution of the reacted fraction (θ )

corresponding to MgH2 decomposition, obtained by integration of H2 peak of commercial MgH2 (AA), and samples irradiated with 1016 ion/cm2 of 45 keV B3+ (B16) and 45 keV N3+ (N16) ions

4. CONCLUSION

The influence of structural changes caused by B3+ and N3+ ion irradiation on sorption properties of MgH2 has been investigated. The irradiation was done using 45 keV and the fluence of 1016 ion/cm2. Numerical simulations were done in order to obtain the depth profile of incident ions and energy loss mechanisms. Sample irradiated with nitrogen ions show the morphology similar to pure MgH2 with agglomerates from 10 μm to 100 μm, but with a lot of cracks leading to quite damaged onion like structure. On the other hand, agglomerates of sample irradiated with boron ions are much smaller which is in agreement with the results of particle size analysis. Both samples show the

monomodal particle size distribution, but with the onset at lower sizes, which is more pronounced for sample irradiated with boron.

Hydrogen desorption properties and kinetics were evaluated from TPD measurements and numerical non isothermal procedure. Using ion irradiation one can control the depth of deposition and the number of defects, and in such way it is possible to unambiguously explain the change in the reaction mechanism. It has been shown that the reaction mechanism depends on quantity and deposition depth of defects given that the reaction mechanism change from phase boundary reaction with spherical symmetry (n = 3) regarding the untreated sample and sample irradiated with boron to phase boundary mechanism with n = 4 for sample irradiated with nitrogen. It has been demonstrated that the changes in near-surface area play the crucial role in hydrogen desorption kinetics. It is also confirmed that there is a possibility to control the thermodynamic parameters by controlling vacancies concentration in the investigated systems.

Acknowledgments

This work has been financially supported by the Ministry of Education, Science and Technology under grant III45012.

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